1. Field of the Invention
The present invention relates to a cathode ray tube (CRT) display apparatus and a control method thereof.
2. Description of the Related Art
In a conventional CRT display apparatus, a fly back transformer (FBT) outputs a voltage of about 26 kV. To supply such a high voltage stably, the FBT commonly uses two types of high-voltage stabilization circuits, one type being a separated-type circuit in which transformation and deflection are separated, and the other type being an incorporated-type circuit in which the transformation and the deflection are integrated.
FIG. 1 is a circuit diagram illustrating an input side of a secondary induction coil of the FBT having the incorporated type high-voltage stabilization circuit in the conventional CRT display apparatus.
As shown in FIG. 1, the conventional CRT display apparatus comprises an FBT 100, a horizontal regulating circuit 110, an auto beamcurrent limit (ABL) adjusting circuit 120, and a vertical regulating circuit 130.
The FBT 100 includes a primary induction coil 101 and a secondary induction coil 102. The secondary induction coil 102 has an output terminal electrically connected to a CRT, and an input terminal 116 electrically connected to the horizontal regulating circuit 110, the ABL adjusting circuit 120, and the vertical regulating circuit 130. In the FBT 100, a voltage applied to the primary induction coil 101 is amplified in the secondary induction coil 102, and then the amplified voltage is supplied to an anode of the CRT. Here, a voltage of about 50 to 180 V is applied from a step-up circuit to the primary induction coil 101, inducing a high voltage of about 25 kV in the secondary induction coil 102.
The horizontal regulating circuit 110 includes resistors R2 and R3, a diode D2, and a micro control unit (MCU) (not shown). When the voltage applied to the input terminal 116 fluctuates according to brightness of a screen, the horizontal regulating circuit 110 provides feedback due to the voltage fluctuation by using passive elements such as the resistors R2 and R3 to a horizontal size (H-size) control part. Further, the MCU compensates a predetermined value for the voltage fluctuation. Here, the compensated value in the MCU is transmitted to the H-size control part according to the voltage fluctuation with an average value based on an operating frequency of a scanning line according to the CRT display apparatus, wherein the average value is taken from experimental data.
The ABL adjusting circuit 120 includes a diode D4, and resistors R8 and R9. The ABL adjusting circuit 120 detects the voltage fluctuation at the input terminal 116 through the resistors R8 and R9 and the diode D4, and controls a beamcurrent being supplied to the CRT.
The vertical regulating circuit 130 comprises resistors R4, R5, R6 and R7 and diodes D1 and D3. The vertical regulating circuit 130 determines a voltage at a node 117 by comparing a voltage of 12 V applied to the resistor R5 with the voltage applied to the input terminal 116, and the voltage at the node 117 is fed back to an H/V processor to generate a current in a vertical deflection coil. Here, the H/V processor 130 regulates an amplitude of the current flowing in the vertical deflection coil, thereby controlling a vertical size (V-size) of the screen.
When the screen is changed in the brightness, electrons colliding with RGB (Red, Green, Blue) pixels are changed in energy. As the brightness of the screen increases, a higher energy of electrons is required, that is, a higher current is required. Because the FBT is supplied with constant power, when the current flowing in the secondary induction coil 102 is high, the voltage applied to the input terminal 116 is decreased.
Thus, the change of the brightness of the screen is determined by sensing the change of the voltage applied to the input terminal 116 of the secondary induction coil 102.
In the incorporated type high-voltage stabilization circuit, when the brightness of the screen increases, the voltage supplied to the anode of the CRT increases but the currents flowing in the horizontal and vertical deflection coils decrease. Further, when the currents flowing in the deflection coils decrease, the horizontal size (H-size) and the vertical size (V-size) of the screen are decreased. Therefore, when a display mode of the screen is altered among a text mode, an internet mode, a game mode, an entertainment mode, etc., the size of the screen suddenly and widely fluctuates. Particularly, in the CRT display apparatus for a monitor provided with a hot key to directly control the brightness of the screen, the size fluctuation of the screen due to the brightness change is more noticeable.
Recently manufactured monitors typically have a brightness of about 120 candela per square meter (cd/m2), which is generally lower than that of a television (TV) screen, so that the monitor cannot display a picture more brightly than the TV. To overcome this problem, a CRT display apparatus having a picture boost function to display a picture with the brightness of about 500 cd/m2 has been developed.
To compensate the H-size fluctuation of the screen due to the brightness change, a sawtooth wave generating circuit is provided with horizontal regulating parts in addition to the horizontal regulating circuit 110 of FIG. 1. Meanwhile, a vertical regulator only senses the voltage change, so that the V-size fluctuation due to the brightness change is not precisely compensated in comparison with the H-size fluctuation. Further, the MCU of the horizontal regulating circuit 110 compensates the H-size of the screen with an average value based on several CRT display apparatuses, so that size-compensation deviation may occur among the CRT display apparatuses.